Within the context of producing natural gas or liquefied natural gas, it is necessary to purify said natural gas, originating from deposits, of a certain number of contaminants, primarily acidic gases such as hydrogen sulphide (H2S) and carbon dioxide (CO2). In particular, carbon dioxide can represent a major part of the gaseous mixture originating from a deposit of natural gas, up to more than 70% (in molar concentration). Several processes are known in the field for making it possible to reduce the carbon dioxide content of the natural gas.
The most usual treatment is based on the use of amine solvents. This method makes possible a separation of the CO2 that is very selective vis-à-vis hydrocarbons; it makes it possible to lower the concentration of CO2 below the threshold of 50 ppm. But this method requires high energy to regenerate the solvent. As a result, it is unsuitable if the original gas has a high concentration of CO2. Moreover, the regeneration is virtually atmospheric, and requires a compression that consumes a lot of energy if a reinjection of the separated CO2 is envisaged (which is to be envisaged more and more routinely in view of the environmental issues).
Another type of treatment is based on the use of semipermeable membranes. The uses of these membranes for gases with an average CO2 content have developed significantly in the last few years. Membrane treatment is advantageous for significant concentrations of CO2 and for a certain range of “feed-to-retentate” partial pressure ratios. However, when the CO2 specifications are relatively low, the associated losses of methane can become considerable. It is also possible to provide several stages of membranes for concentrating the CO2 in the permeate, which makes it necessary to provide intermediate compressions of the permeate. The reinjection of the CO2, if sought, requires an additional compression, from the low pressure of the final permeate, which further increases the energy bill for this type of process.
Cryogenic processes constitute another type of treatment. The higher the concentration of CO2 in the original gas, the greater their advantage in terms of energy. An example of a cryogenic process is shown in U.S. Pat. No. 4,152,129. However, due to the possible crystallization of the CO2 and/or the critical conditions at the head of the column, such a process does not allow stringent CO2 requirements to be met. A finishing treatment, for example of the amines type, is therefore essential if a strict CO2 specification is required.
Certain variants of cryogenic treatment have been presented more recently, in particular the process called “CFZ” (“Controlled Freeze Zone”), the particular feature of which is to allow a crystallization of the CO2 in the problematic zone of the column, which makes it possible to envisage very high specifications with very low treatment temperatures (about −90° or even −110° C.). On this point, reference may be made for example to U.S. Pat. No. 4,533,372.
Another variant of cryogenic treatment has been developed by Cool Energy Limited. This process, called “CryoCell”, makes it possible, by means of a cryogenic separation step, to meet specifications of about 2 to 3% CO2, starting from a gas pretreated by cryogenic distillation, or directly for crude gases with an average concentration of CO2 (typically 25 to 35%). This process uses a liquefaction of the gas under pressure, then an expansion of the fluid which creates an intense cold and a partial crystallization of the CO2. The liquid and solid fractions are recovered in a flask designed for certain methods of application, keeping the bottom temperature in the liquid range. WO 2007/030888, WO 2008/095258 and WO 2009/144275 illustrate this technique.
Another variant of cryogenic treatment is constituted by the family of so-called “Ryan Holmes” processes. These processes, which make possible a fairly complete recovery of the C3+ hydrocarbons, use 3 or 4 distillation columns, depending on the nature of the gas, and as a result prove to be relatively complex and costly in terms of investment and consumption.
A drawback of these cryogenic methods is that they separate the components according to their volatility and therefore, with the liquid CO2, trap virtually all of the C3+ hydrocarbons contained in the natural gas. This constitutes a sometimes very great handicap depending on the composition of the gas. It is estimated that 8 to 15% by mass of the hydrocarbons are generally lost when a separation of the CO2 by distillation is implemented; furthermore, the majority of the hydrocarbons lost are hydrocarbons with an intermediate molar mass, therefore the most valuable.
WO 99/01707 relates to a variant of the process called “CFZ”, in which some of the stream of liquid CO2 recovered at the foot of the distillation column is expanded, then used to cool the natural gas before it enters the distillation column in two successive heat exchangers. Between the two heat exchangers, the stream of CO2 undergoes a gas/liquid separation, only the liquid portion being expanded then guided to the second heat exchanger (the gaseous portion being compressed before finally being removed). At the outlet of the second heat exchanger, another gas/liquid separation is provided: the gaseous phase is compressed before finally being removed, while the liquid phase provides a recovery of the condensates trapped in the stream of CO2.
This technique makes it possible to limit the hydrocarbon losses in the stream of liquid CO2 and could be applied to any process for cryogenically separating the CO2 which traps C3+ hydrocarbons in the liquid CO2. On the other hand, a drawback of the technique proposed in this document is that the composition of the stream (mostly CO2) in the successive heat exchangers varies, the stream becoming progressively richer in heavy fractions. This leads to an increased risk of crystallization, in particular of the paraffinic hydrocarbons, and particularly in the last heat exchanger in the cold cycle, the temperature of which is the lowest. This is why the document provides the alternative of a rectification column for the natural gas at the inlet of the plant in order to avoid these problems, so as to remove some of the heavy compounds upstream. This method is extremely complex and difficult to implement, since it requires an additional fractionation of all of the gas.
There is therefore a real need to develop a treatment that makes it possible to effectively reduce the hydrocarbon losses for these types of cryogenic separation of CO2, in a manner that is simple to implement.